22a – Pathophysiology of Anesthesia: CV Flashcards
What does the medulla do?
- Controls sympathetic and parasympathetic nervous system output
- Receives feedback from various systems to maintain appropriate CO
Sympathetic nervous system
- Release catecholamines (E, NE)
- Changes mainly affects arteries, arterioles and great veins
Parasympathetic nervous system
- Release ACh
- Changes mainly affect cardiac rate and rhythm
What is the ultra-short acting control?
- Basal vasomotor tone
- *not really affected by anesthesia
o Can be disrupted by disease processes - Autoregulation and endothelium derived factors
Autoregulation
- Organs sense tissue O2 demands=increase/decrease blood flow to match
- Sense accumulation of K, H, Co2, adenosine, lactate
What are the endothelial derived local vasodilators?
- NO
- PGI2
What are the endothelial derived local vasoconstrictors?
- Endothelins
- Thomboxanes (A2)
What is the short term control?
- Reflexes
o Baroreceptor
o Chemoreceptor
o Bainbridge reflex
o Frank-starling relationship - *important for anesthesia
o Blunted by inhaled and injectable anesthesia drugs
Baroreceptor reflex
- In carotid sinus and aortic arch
- Sense changes in BP and result in changes to HR
Chemoreceptor reflex
- Carotid and aortic bodies
o Perfusion, CO2 and O2 blood levels and blood pH - Feedback to brainstem to adjust ventilation and sympathetic activity
Bainbridge reflex
- Stretch receptors in right atrium
- Sense increased pressure in R. atrium (increase venous return) and result in INCREASED HR
Frank-Starling relationship
- Increased venous return stretches myocardium
- Leads to increased myocardial contractility and increased SV
What is perfusion pressure?
- What provides adequate blood flow (perfusion) to tissues
o MAP – pressure withing tissues
o MAP>60mmHG=provides adequate tisse blood flow to major organ systems - *changes in perfusion pressure leads to IMMEDIATE effects on organ function
What is the cerebral perfusion pressure equation?
- MAP – intracranial pressure
What is the renal perfusion pressure equation?
- MAP – glomerular capillary pressure
What is the equation for the autoregulation zone? (brain and kidney perfusion)
- =range of MAP where organs maintain perfusion pressure to OPTIMIZE tissue perfusion
- *goal=maintain MAP>60mmHG
o If lower than 60mmHG=outside autoregulatory zone, perfusion becomes dependant on systemic BP
Coronary artery perfusion
- Myocardial perfusion occurs in DIASTOLE=depends on diastolic arterial pressure (DAP)
- *coronary artery perfusion=DAP – R. atrial pressure
- *goal=maintain MAP>40mmHg
o If lower=reduced CAP leading to MYOCARDIAL ISCHEMIA
Anesthesia effects: when a vasodilator
- Reduced systemic vascular resistance (SVR)
o Inhale=dose dependent
o Acepromazine=alpha1 adrenergic blockage
o Meperidine/morphine IV=histamine release
o Propofol and alfaxalone=preferential VENODILATION (after IV bolus) - *effects are additive when used together
- *can lead to HYPOTENSION during anesthesia
Anesthesia effects: when a vasoconstrictor
- Increase SVR
o Alpha2 adernergic agonists=direct action on PERIPHERAL alpha-2 receptors
o Ketamine and NO=sympathomimetic action that realeases endogenous catecholamines - *can lead to HYPERTENSION
o Can be seen in PREmedication, but reduced by inhaled anesthetics
What are the drugs that have minimal vascular effects? (2)
- Opioids
- Benzodiazepines
What do epidurals do to vasculature?
- Local anesthetic can travel to thoracic region of spinal cord=BLOCK sympathetic vasomotor nerves
o *RESULT=reduced sympathetic control on vasomotor tone=vasodilation (decreased SVR)
o Compounded by other vasodilatory anesthetic drugs - **effects are TEMPORARY but extra care should be taken with large animals
How can you avoid vasodilation from epidurals?
- *careful administration
- Lowest volume required to provided adequate nerve blockade
- SLOW injection speed=reduced injection pressure to slow forward spread
What are some drugs that decrease myocardial contractility?
- Inhaled anesthetics, alfaxalone, propofol=decrease Ca availability
- Acepromazie (mild) and alpha2-adrenergic agonists
- Dose-dependent effects
What are some states that reduce contractility?
- Hypoxemia
- Depleted catecholamines (sepsis)
- Acidosis (pH<7.0)
- Electrolyte imbalances (increase K, decreased Ca)
- Hypovolemia (Starling’s law)
What are the values for prehypertension?
- SAP 140-150mmHg
What are the values for hypertension?
- SAP>160-179mmHg
What are the values for severe hypertension?
- SAP>180mmHg
What can acute severe hypertension lead to?
- Decreased CO
- Edema
- Hemorrhage (brain/lungs)
What does sustained hypertension result in?
- Myocardial remodelling
- Retinopathy or retinal detachment
- Encephalopathy
- Renal disease
What is the treatment for hypertension?
- NEED TO ID underlying reason
o *Sympathetic activation: light plane anesthesia, nociception, hypercapnia, hypoxemia
o Diseases: chronic kidney disease, hyperadrenocorticism
What are the values of hypotension (SAP, MAP, DAP)
- SAP<80mmHg
- MAP<60mmHg (large animals MAP<70mmHg, care more about skeletal muscle perfusion)
- DAP<40mmHg
- *more common
What is significant hypotension?
- Severity
o Mild/moderate: MAP<60mmHg
o Severe: MAP<35-45mmHg - Time frame
o How quick?
o How long?
What are the sequelae of moderate, severe or prolonged hypotension (>5-15mins)
- Renal damage and poor urine production
- Hepatic and gastrointestinal damage (necrosis)
- Increased lactate concentration
- Myocardial ischemia, arrhythmias and cardia arrest
What is HR and rhythm controlled by?
- Balance of autonomic nervous system
o PS: vagus=release Ach=decreased HR
Receptors: atria, conducting tissue
o S: T1-T4 spinal nerves and cervical ganglia from cardiac nerves
Release NE=increase HR
B1 receptors: chronotropic, inotropic, dromotropic
B2 rectors (atria): chronotropic
What is bradycardia?
- *less than half of resting HR
o Dogs: <50bpm
o Cats: <90bpm
What are bradyarrhythmias?
- Increase PS tone
o Usually drug or disease induced
What is tachycardia?
- Dogs >180bpm
- Cats >200bpm
- *species differences
What are tachyarrhythmias?
- Increased sympathetic tone
o Nociception
o Anxiety
o Hypercapnia
o Hypoxemia
o Hypovolemia
o Drugs
What does xylazine and ketamine do to heart rhythm?
- Stimulate alpha1 and beta1=tachyarrhythmias
What do opioids and alpha2-adrenegic agonist do to hearth rhythm?
- Increase vagal (PS) tone=bradyarrhythmias
What does acepromazine do to heart rhythm?
- Anti-arrhythmic=alpha1 adrenergic blockage
What drugs have minimal to NO effect on heart rhythm?
- Inhalant anesthetics (until deep plane of anesthesia)
- Propofol, alfaxalone
- Benzodiazepine
